Optionally, the
disks in an ASM group can be mirrored to provide additional redundancy and performance benefits.
Using ASM provides a number of other benefits. In many cases, an ASM instance with a number
of physical disks can be used instead of a third-party volume manager or network-attached storage
(NAS) subsystem. As an added benefit over volume managers, ASM maintenance operations do
not require a shutdown of the database if a disk needs to be added or removed from a disk group.
In the next few sections, we??™ll delve further into how ASM works, with an example of how to
create a database object using ASM.
Disk Group Redundancy
A disk group in ASM is a collection of one or more ASM disks managed as a single entity. Disks
can be added or removed from a disk group without shutting down the database. Whenever a
180 Oracle Database 11g DBA Handbook
disk is added or removed, ASM automatically rebalances the datafiles on the disks to maximize
redundancy and I/O performance.
In addition to the advantages of high redundancy, a disk group can be used by more than one
database. This helps to maximize the investment in physical disk drives by easily reallocating disk
space among several databases whose disk space needs may change over the course of a day or
the course of a year.
As I explained in Chapter 4, the three types of disk groups are normal redundancy, high
redundancy, and external redundancy. The normal-redundancy and high-redundancy groups
require that ASM provide the redundancy for files stored in the group.
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